Turbine shroud structure



y 21, 1934 w. B. MOYER 3,141, 651

TURBINE SHROUD STRUCTURE Filed Aug. 13, 1962 2 Sheet-Sheet 1 FIG.3

INVENTOR WAYNE B. MOYER, BY 4% Z.

HIS ATTORNEY.

July 21, 1964 4 w. B. MOYER INVENTOR B. MOYER, m

HIS ATTORNEY.

WAYNE BY M. F.

United States Patent 3,141,651 TURBWE SHRUUD STRUCTURE Wayne B. Moyer, Schenectady, N.Y., assignor to General Electric Company, a corporation of New York Filed Aug. 13, 1962, Ser. No. 216,443 Claims. (Cl. 25339) This invention relates to an improved stationary shroud structure for use with rotating turbine blades operating at high temperatures.

Rotating turbomachines operating at very high temperatures, such as gas turbines, require special provisions to maintain proper clearances between rotating and stationary parts at all temperature conditions. One example of this is the stationary shroud structure which forms close clearances with the tips of the rotating turbine blades to prevent the leakage of gas between the tips of the blades and the casing. Inasmuch as large temperature changes may occur rapidly in gas turbines, the stationary shroud structure must respond quickly to such temperature changes and expand radially at substantially the same rate as the turbine blades in order to prevent rubbing which might damage the blades or shroud.

It is well known that the actual shroud clearance pieces which form the close clearances with the turbine blades may be arcuate segments so as to allow for thermal expansion and contraction of the shroud segments without binding or distortion. However, the shroud segments must be supported by a member called a shroud holder which will seek to hold the shroud segments coaxial with the turbine rotor and also provide the proper control of blade tip clearance under all temperature conditions.

One approach to this problem has been to make the shroud holder a continuous ring which forms a part of the turbine casing itself. Although such a continuous shroud holder, which is also a section of the casing, gives relatively good uniformity of radial spacing between the shroud segments and the blades, some portions of the casing must be made in upper and lower halves to permit assembly and inspection. Therefore, the foregoing construction requires that the turbine casing be made up of several sections which are bolted on either side of this type of shroud holder. Also, since the casing must be relatively heavy to contain the pressure of the operating fluid, to provide for bolting flanges, and to support the nozzle diaphragms, it is slow to respond to temperature changes. Also the heat from the shroud segments is conducted directly into the casing wall, thus requiring (with higher turbine inlet temperatures) special casing alloys to withstand these temperatures.

Another approach has been to make a separate split ring shroud holder in two halves which are bolted or otherwise secured together for either use inside of or as an integral part of a split turbine casing, Le, a casing manufactured with mating upper and lower semi-cylindrical halves. Although this type of shroud holder may be separate from the casing and its temperature may thus be isolated somewhat from the casing itself, difiiculties have been encountered with shroud holders made in two or more pieces. This is because they tend to distort into an elliptical shape when subjected to varying radial ther mal gradients, rather than remaining round as does a continuous ring.

Accordingly, one object of the present invention is to provide an improved shroud holder which will adjust rapidly to temperature changes while holding the shroud segments with uniform radial bucket tip clearance.

Another object of the present invention is to provide an improved shroud holder which can expand and contract radially with respect to a split turbine casing while all parts about its circumference are maintained at a uniform distance from the rotor axis.

3,l4l,65l Patented July 21, 1964 Still another object of the invention is to provide an improved shroud holder and mounting structure therefor, suitable for use in a split turbine casing, which also permits adjustment of radial tip clearances without adjusting relative positions of the rotor and main casing.

Yet another object of the invention is a shroud holder with means for providing cooling thereof and isolation of the temperature of the motive fluid from the turbine casing wall.

Another object of the invention is to provide an improved shroud structure which minimizes rubbing by the turbine buckets on the shroud segments.

A more specific object of the invention is to provide an improved continuous ring shroud holder for use in a split turbine casing which will respond to temperature variations and maintain uniform tip clearance control to provide an improved rotating seal between a stationary shroud and rotating blades in a gas turbine.

The subject matter which is regarded as the invention is particularly pointed out and distinctly claimed in the concluding portion of the specification. The invention, however, both as to organization and method of practice, together with further objects and advantages thereof, may best be understood by reference to the following descrip tion, taken in connection with the accompanying drawings in which:

FIG. 1 is a perspective view of portions of a gas turbine rotor and easing incorporating the stationary shroud structure of the invention,

FIG. 2 is a cross sectional view through the shroud structure taken at an intermediate point on the casing periphery,

FIG. 3 is a plan view of an arcuate shroud segment suitable for use with the improved shroud structure,

FIG. 4 is a partial View of shroud holder and shroud segments looking in an axial direction and showing the method of mounting on the shroud holder,

FIG. 5 is a plan view, partly in section, taken at the dividing joint of the split casing, and

FIG. 6 is an axial view, in section, at the horizontal joint of the casing, taken along line VIVI of FIG. 5.

Generally stated, the invention is practiced by providing a shroud holder which is a continuous circular ring adapted for mounting arcuate shroud segments thereon and held in a split turbine casing coaxial with the turbine rotor and arranged to expand and contract radially relative to the casing. The continuous ring is placed around the turbine rotor before the rotor is inserted into the lower casing half.

Referring now to FIG. 1 of the drawing, a turbine casing shown generally as 1 includes an upper semi-cylindrical half 2, and a lower mating semi-cylindrical half 3, secured together by radial flanges 2a, 3a respectively along a horizontal joint 4. The casing wall 5 is relatively thick and may be relatively irregular in shape and have various attachments thereon necessary to the operation of the turbine, but these details are not shown since they are unnecessary to the understanding of the present invention.

Disposed inside turbine casing 1 is a turbine rotor, shown generally as 6, which has a shaft 7 rotatably sup ported in bearings (not shown) to hold rotor 6 coaxial with walls 5 of the casing. Shaft 7 carries a turbine wheel 8, which may either be formed integrally with shaft 7 or shrunk onto shaft 7. Wheel 8 carries on its outer rim a number of radially extending turbine blades 9, which are attached in conventional fashion, these details also being omitted from the drawing. The tips of blades 9 are open and, in order to prevent leakage of gas between the tips of blades 9 and the casing wall 5, a shroud structure, shown generally as Ill, is provided.

Shroud structure 10 includes a shroud holder 11, which J is a continuous circular channel-shaped ring, and a num ber of arcuate shroud segments 12 held on the radially inner part of ring 11.

Wall of the casing 1 has one or more circumferential radially-extending tongues 13 formed on the interior of both top half 2 and bottom half 3. Tongue 13 prevents axial movement of shroud holder 11, and supports the shroud holder for radial movement relative to casing walls 5. The turbine rotor 6 may have more than one stage of blades. FIG. 1 also shows the details of part of a tongue 13, for a second turbine stage (not shown) and its shroud structure (also not shown).

Shroud holder 11 is held coaxial with the turbine casing wall 5 by means of horizontally extending pins 14 and vertically extending pins 15, the vertical pin at the top of casing 1 not being shown in the drawing. Pins 14 are secured by bolts 16 in recesses 17 cut in mating flanges 2a, 3a, so that a proper seal can be maintained along joint 4. Pins extend radially inwardly through recesses 18 cut in diametrically opposite parts of the casing. Recesses 17, 18 are arranged on the centerline of tongue 13, and it will be observed that the ends of pins 14, 15 extend radially inward beyond the inner edge of tongue 13. The ends of pins 14, 15 hold the shroud assembly 111 between pairs of spaced rollers 19 disposed on the shroud holder 11.

Referring now to the cross section of FIG. 2, which is taken through the shroud holder and casing at a point circumferentially spaced from either of the holding pins 14, 15, it will be observed that shroud holder 11 is channel-shaped and has radially extending legs 20 closely iit ting on either side of circumferential tongue 13. A radial clearance space 21 between casing wall 5 and the ends of legs 20 allows shroud holder 11 to expand radially with respect to casing wall 5 before engaging it.

The interior of shroud holder 11 may be cooled by means of air from a sealed channel-shaped manifold 22 bolted or welded to the exterior of casing 5, and fed with air bled from the turbine compressor through a pipe 23 (see also FIG. 1). The cooling air flows inwardly through a conduit 13a in tongue 13 and, after cooling the interior of shroud holder 11, mingles with the gas stream by flowing out through a conduit 211a in the leg 20.

The radially inner portion of shroud holder 11 includes circumferential grooves 24 which serve to provide a T- shaped support for segments 12. There is also a central circumferential groove 25 which receives a small central tongue 26 on the shroud segment 12 to support the shroud against axial movement and provide a gas seal. Shroud segments 12 include lips 27 which extend axially inward to fit in groove 24, and to hold the shroud against radial movement on shroud holder 11. The radial spacing between shroud 12 and shroud holder 11 provides room for insulating pads 28 which reduce the conduction of heat from shroud 12 to shroud holder 11.

Reference to FIG. 3 of the drawing will illustrate more clearly the structure of shroud segment 12. It is seen that shroud 12 includes a central sealing portion 29, which forms close clearances with the turbine blades 9. Shroud 12 also has axially spaced integral radial walls 311, which are arcuate together with the central portion 29. Axially extending tabs 27 on the four corners of shroud 12 extend inward for a short length along the shroud, in order to support the shroud from the shroud holder, as may be seen by reference to FIG. 4 of the drawing.

In FIG. 4, several shrouds 12 are seen disposed on a portion of shroud holder 11. Adjacent shrouds are circumferentially spaced with a small clearance gap 31 to allow for circumferential expansion of the shrouds on shroud holder 11. It will be observed that a number of radial notches 32 are provided in shroud holder 11 which connect with circumferential groove 24.

In order to mount a shroud 12 on ring 11, it is displaced circumferentially one-half a shroud length from the position shown in FIG. 4, and moved radially out- Ward so that lips 27 will pass through notches 32 into the circumferential groove 24. Thereafter they are. moved one-half a shroud length to the position shown, whereupon lips 27 prevent radial removal of the shrouds. In order to prevent shifting of the shrouds in a circumferential direction on ring 11, two set screws 33 are inserted which project inwardly and prevent movement by abutting against the sides of notches 32.

The tabs 27 serve to support the shroud segment 12 at its four corners rather than at previously suggested support points. It is known that thermal gradients across. such a shroud segment will cause it to curl away from the heat. This has caused rubbing in the middle of the shroud and breakage of previous centerline tabs. By designing the present shroud with the proper proportions, minimizing the radial thermal gradient across it with in sulating pads 28 (see FIG. 2) and supporting it at the four outer corners, the radial clearance variations between shrouds and blade tips, with varying temperature, are substantially reduced.

As stated previously, shroud holder 11 may move radially to a limited degree because of the radial spacing 21 between the ends of legs 20 and the wall 5 of the casing. Shroud holder 11 is additionally held coaxial with the axes of the casing and rotor by means of projecting pins, such as pin 14, at the horizontal joint illustrated in FIGS. 5 and 6. There it will be seen that the head 14a of the pin is bolted into recess 17, and that pin 14 has an extending leg 14]). Leg 14b has a key formed on its lower side which fits tightly in a radial keyway 34 connecting with recess 17. Leg 14b has a further radially extending portion 14d, which extends beyond the end of tongue 13 between legs 21, of the shroud holder 11. The shroud holder 11 is held coaxial with the rotor by means pro viding reduced friction toward radial movement of the shroud holder. In the embodiment shown, support is provided by a pair of rollers 19, which extend axially between legs 20 of the shroud holder and fit tightly on either side of extension 14d. Thus the rollers 19 allow the shroud holder 11 to move radially, and also, when employed with other pins and rollers, maintain the shroud holder 11 coaxial with the axis of casing 1 in a manner well known in the art. It will be apparent that, in order to hold the shroud ring 11 coaxial, there must be at least three such pairs of rollers 19. However, as a practical matter, four sets of rollers and projecting pins are employed, two of these being located on the horizontal joint and two projecting vertically through the upper and lower halves of the turbine casing.

The operation of the invention will be apparent from the following description. Since the mass of shroud holder 11 is much less than that of the outer casing wall, it can respond to temperature variations fairly quickly and can therefore expand or contract as necessary in a radial direction. Since shroud holder 11 is a continuous ring, it will remain a true circle and not distort as has been the case with prior art shroud holders where they were manufactored in two or more arcuate sections. During radial expansion and contraction, the friction of movement is reduced by rollers 19. Clearance for movement is provided by the spacing between shroud holder 11 and the casing wall, so that legs 21! can slide on either side of circumferential tongue 13. The cooperating rollers and pins keep the shroud holder 11 coaxial during expansion and contraction, since the horizontal pins 14 prevent upward or downward movement and the vertical pins 15 prevent side-to-side movement.

The improved shroud holder allows adjustment of ra dial tip clearance by means of the support shown without changing the relative positions of rotor and casing. Although every effort is made during manufacture and assembly to reduce eccentricity between casing and rotor, so that they are absolutely coaxial, unavoidable manufacturing variations may cause the rotor to be slightly misaligned within the casing. Also, split casings tend to become slightly elliptical as stresses are relieved under use at elevated temperatures. The shroud holder 11 can be adjusted to correct these variations and to bring the shroud segments ito coaxial alignment with the rotor, by shimming or grinding off the horizontal and vertical pins 14, 15.

The method of assembly of the improved shroud structure is as follows. With the top half 2 of the casing removed, the shroud segments 12 are placed and secured on shroud holder 11 in the manner described. Then the shroud ring 11 is slid axially onto the rotor and temporarily held in place while the rotor 6 is lowered into the lower half 3 of the casing. The lower pin 15 and the horizontal pins 14 may then be bolted in place. Next, the upper half 2 of the casing is placed and secured to the lower half, whereafter the upper vertical pin, like 15 (not shown), may be inserted and bolted in place in the manner of the lower vertical pin 15.

If desired, the shroud holder 11 may be cooled by compressor air with a controlled rate of flow through conduit 23, so that the thermal response of the shroud ring 11 in a radial direction corresponds to that of rotor 6.

While continuous ring shroud holders are not new, these have been parts of the casing in the past and sometimes caused special alloys to be required for the casings to withstand the higher temperatures. The improved shroud holder described herein in continuous, yet is substantially thermally isolated from the casing so that lower alloy casing materials can be employed.

While there has been described herein what is at present considered to be the preferred embodiment of the invention, it will be understood that various other modifications may be made therein, and it is intended to cover in the appended claims all such modifications as fall within the true spirit and scope of the invention.

What I claim as new and desire to secure by Letters Patent of the United States is:

1. A rotor having a plurality of radially extending blades, a split stator casing having upper and lower halves enclosing said rotor, said stator casing defining a circumferential radially extending tongue, a shroud holder disposed within said casing comprising a continuous circular channel-shaped ring having legs extending radially outward on either side of said tongue, said legs being radially spaced from said casing wall to provide relative radial movement with respect thereto, means holding said continuous shroud holder ring coaxial in said casing, and a plurality of arcuate shroud segments disposed end to end around said ring and forming close clearances with the tips of the rotor blades.

2. The combination according to claim 1, including conduit means conducting cooling fluid through said tongue to the space between the legs of said shroud holder.

3. The combination according to claim 1 wherein said shroud holder ring defines axially directed circumferential grooves on either side thereof near its radially innermost part, and wherein said shroud segments include axially directed tabs on each corner thereof, which tabs are disposed in said grooves to support the shroud segments from their corners.

4. A rotor having a plurality of radially extending blades, a split stator casing having upper and lower halves enclosing said rotor, said casing defining an inwardly extending circumferential tongue, a shroud holder disposed within said casing comprising a continuous circular channel-shaped ring with legs extending radially outward on either side of said tongue and spaced from the casing, at least three circumferentially spaced pairs of rollers extending axially between the legs of said shroud holder ring, pin means for each of said pairs of rollers adapted to be attached to said casing and extending radially beyond the ends of said tongue, said pin means being circumferentially spaced and each extending between two rollers to hold the shroud holder coaxial with said casing, and a plurality of shroud segments disposed end to end around said ring and forming close clearances with the tips of the rotor blades.

5. A rotor having a plurality of radially extending blades, a split stator casing having upper and lower halves enclosing said rotor, each stator half having flanges adapted to be connected at a horizontal joint, a shroud holder disposed within said casing comprising a continuous circular channel-shaped ring having continuous axially spaced legs extending radially outward toward the walls of said casing, said casing defining a circumferential radially extending tongue disposed between and radially spaced from the innermost part of the outwardly facing surface between the legs of said shroud holder, at least four pairs of axially extending rollers extending between said spaced legs of the shroud holder and circumferentially spaced substantially degrees apart on said shroud holder, first and second horizontal pin means disposed within said casing flanges and adapted to be secured to said flanges on opposite sides of the casing, said first and second pin means having portions extending radially inward beyond said tongue, third and fourth vertical pin means extending radially through the walls of the upper and lower casing halves respectively, said third and fourth pin means including portions extending radially inward beyond the tongue, the extending portions of said first, second, third and fourth pin means being received between the rollers of each pair of shroud holder rollers respectively, and a plurality of shroud segments disposed end to end around said ring and forming close clearances with the tips of the rotor blades.

References Cited in the file of this patent UNITED STATES PATENTS 2,447,942 Imbert et al. Aug. 24, 1948 2,494,178 Imbert Jan. 10, 1950 2,888,240 Fleischmann et al. May 26, 1959 OTHER REFERENCES German printed application, 1,022,237, I an. 9, 1958. 

1. A ROTOR HAVING A PLURALITY OF RADIALLY EXTENDING BLADES, A SPLIT STATOR CASING HAVING UPPER AND LOWER HALVES ENCLOSING SAID ROTOR, SAID STATOR CASING DEFINING A CIRCUMFERENTIAL RADIALLY EXTENDING TONGUE, A SHROUD HOLDER DISPOSED WITHIN SAID CASING COMPRISING A CONTINUOUS CIRCULAR CHANNEL-SHAPED RING HAVING LEGS EXTENDING RADIALLY OUTWARD ON EITHER SIDE OF SAID TONGUE, SAID LEGS BEING RADIALLY SPACED FROM SAID CASING WALL TO PROVIDE RELATIVE RADIAL MOVEMENT WITH RESPECT THERETO, MEANS HOLDING SAID CONTINUOUS SHROUD HOLDER RING COAXIAL IN SAID CASING, AND A PLURALITY OF ARCUATE SHROUD SEGMENTS DISPOSED END TO END AROUND SAID RING AND FORMING CLOSE CLEARANCES WITH THE TIPS OF THE ROTOR BLADES. 